1. Field of the Invention
The present invention relates to a diffraction element, objective lens unit, optical pickup and optical disc apparatus, and is preferably applied to an optical disc device, for example.
2. Description of Related Art
There is an optical disc device that supports a plurality of types of optical discs: Compact Disc (CD), Digital Versatile Disc (DVD) and “Blu-ray Disc (Registered Trademark)” (BD). The wavelengths of laser beam vary according to the types of optical discs.
As an optical pickup for the optical disc device, there is one capable of collecting different wavelengths of laser beams by a diffraction element and a subsequent objective lens: Considering different focal distances of the optical discs, the diffraction element only diffracts particular wavelengths of laser beams while laser beams of other wavelengths passes through it without being diffracted.
This kind of diffraction element includes a step-like diffraction pattern made from a resin, and a plane cover layer made from the other resin to cover the diffraction pattern (see Jpn. Pat. Laid-open Publication No. 2005-302270, for example).